1. Field of the Invention
This invention relates to electrical measurements of semiconductor properties during the manufacturing process of integrated circuits and in particular to means for measuring the dimensions (e.g. lengths and widths) of structures in an integrated circuit.
2. Description of Prior Art
In-line measurements are an important method of controlling semiconductor fabrication processes. Such measurements enable a quick response to variations of critical fabrication process steps. (As used herein, "inline measurements" are measurements made during the fabrication process, i.e. prior to device completion.) Also the feedback from in-line measurements reduces the number of products damaged by defective processing through the fast detection of problems. One of the most important parameters to measure during production is the width of lines which form electrical elements in semiconductor devices.
A common sequence in semiconductor manufacturing is a deposition of a thin layer of a material such as polysilicon on a wafer followed by a deposition of a photoresist layer on top of the polysilicon. The photoresist is then exposed to light through a patterned photomask.
The transparent portion of the photomask permits passage of light which interacts with the photoresist. The photoresist is then developed and etched using a chemical that removes the exposed portion of the photoresist, thereby exposing a portion of the underlying polysilicon. Then, the exposed portion of the polysilicon layer is etched away, e.g. by reactive ion etching. It is very crucial to accurately control the width of the resulting polysilicon structure.
To achieve the required degree of control, both the photoresist and the final polysilicon line widths are monitored on a regular basis during the fabrication process. Currently in-line width measurements use optical techniques. However, there are several problems with such optical techniques, e.g. they cannot be used to accurately measure small structures with great precision. Also, optical measurements are not very reproducible, and are dependent on the person performing the measurements. Optical measurement techniques are also slow, difficult to automate, and it is difficult to integrate the optical data into a computerized data base.
It is also known in the art to perform electrical measurements on integrated circuits. Most electrical measurements are done only following fabrication, and can be done only on conductive structures. Further, electrical measurements on such structures can be done only following irreversible definition of the conductive layer used to build the structure. Further, the conductive layer to be measured cannot be covered by any nonconductive material such as oxide during the measurement.
Another very important parameter to control in semiconductor fabrication is the exact profile of lines being etched. If, for example, polysilicon is used as a gate electrode in an MOS transistor, the channel length of the transistor is determined by the profile of the polysilicon lines. Further the planarity of the wafer surface is controlled by the profile of the structure formed therein. Currently line profiles are monitored by breaking wafers and examining the cross-section profile with a microscope.